Last century witnessed multiple improvements in optical spectrometer design and dramatic reduction in size. As a result, spectrometers have moved from optical laboratories to industrial, field, aerospace and other areas of application where compactness, ruggedness, reliability, and low cost are crucially important.
Several companies supply compact spectrometers of traditional configuration for ultraviolet, visible, and near-infrared spectral bands. For example, two such companies are Hamamatsu Photonics Co., Ltd. and Ocean Optics (see links below); however, new achievements in nanotechnology make it possible to develop even smaller spectral devices.
For example, U.S. Pat. No. 4,923,271 to Henry et al (“Henry”) issued on May 8, 1990 describes an optical multiplexer/demultiplexer comprising cascaded elliptic Bragg reflectors (gratings). All gratings are formed by means of microlithography in a planar waveguide. Each grating is tuned to a definite light wavelength corresponding to one of the working channels. The gratings have one common focal point but different elliptical ties so that the location of the remaining focus can be chosen to provide adequate spacing between input and output. Preferably, the plurality of elliptical Bragg gratings is ordered such that the grating associated with the shortest wavelength is positioned closest to the input of the device. In principle, this type of optical chip can be used as a spectral device for limited amount of wavelengths; however, extending this type of optical chip to a large number of channels is not feasible, and this is the main disadvantage of the approach. The gratings are separated spatially for sequential processing of light. As the number of channels and correspondingly the number of wavelengths to be processed grows, the size of the device increases, the path of light to the remote gratings grows, and, consequently, intrinsic losses grow as well. Also, building large devices is difficult and expensive due to limited precision of the lithographic process and limited uniformity of the waveguide used for gratings.
A new approach to spectral planar integrated devices is based on superposition of multiple sub-gratings on the same planar area. Each sub-grating resonates to a fixed wavelength, but a super-grating comprising many sub-gratings can be deployed as a spectral instrument. Several devices and systems based on this new approach are disclosed in several pending U.S. Patent Applications such as U.S. patent application Ser. No. 10/405,160 filed by V. Yankov et al on Apr. 2, 2003 entitled “Planar holographic multiplexer/demultiplexer”; U.S. patent application Ser. No. 10/137,152 filed by S. Babin et al on May 2, 2002 entitled “Photonic multi-bandgap lightwave device and methods for manufacturing thereof”; U.S. patent application Ser. No. 10/167,773 filed by L. Polonskiy et al. on Jun. 11, 2002 entitled “Integrating elements for optical fiber communication.” However, none of these publications discloses how the new approach can be introduced into the structure of a spectrometer.
The overlaying of multiple sub-gratings for optical multiplexer/demultiplexer applications was further developed by Vladimir Yankov et al as disclosed in “Multiwavelength Bragg Gratings and Their Application to Optical MUX/DEMUX Devices,” Photonic Technology Letters, vol. 15, pp. 410-412, 2003.
Based on the above principle, several optical systems were patented by Thomas Mossberg et al (see U.S. Pat. No. 7,120,334 issued on Oct. 10, 2006 entitled “Optical Resonator Formed in a Planar Optical Waveguide with Distributed Optical Structures.” However, the inter-laser cavity spectrometer proposed by T. Mossberg in U.S. Pat. No. 7,120,334 has a narrow band limited by laser spectral properties and a cavity-free spectral range, works only on the absorption principle, and analyzes only liquids. The remaining two patents do not teach a compact spectrometer.
S. Grabarnik et al reported information on a miniature spectrometer with a volume of 0.135 cm3 and dimensions of 3×3×11 mm mounted directly on the surface of a charge-coupled device (CCD) sensor (see Optics Express, Vol. 15, No. 6, pp. 3581-3588, 2007). The spectrometer is formed by two flat diffraction gratings that are designed to perform both the dispersion and imaging functions, eliminating the need for spherical optics. Two separate parts of the device were fabricated with single-mask 1/Jm lithography on a single glass wafer. The wafer was diced, and the device was assembled and directly mounted onto a CCD sensor. The resolution of 3 nm, spectral range of 450 to 750 nm, and the optical throughput of ˜9% were measured to be in a complete agreement with the model used for development of the device.
In “Investigation of the use of CCDs as high-resolution position-sensitive detectors of ionizing radiation (Lawrence Berkeley Laboratory)”, A. Bross reported successful use of charge-coupled devices (CCDs) as analog shift registers, optical imagers, and high-density memories. In fact, the device comprises a CCD Planar spectrometer operable in either one- or two-dimensional modes.
A common disadvantage of the above-described known optical spectrometers is their relatively large dimensions, and the applicants are unaware of the existence of miniature optical planar spectrometers designed and operating on the principle of digital planar holography.